Getting Ready for Spring a Few Days Earlier

Getting Ready for Spring

This weekend I plan to work in the garden, start getting dirt ready for spring. Here is the map of the hardiness zones from the U.S. National Arboretum. This map is from 1990, and it is a guide on when to plant based on frost dates.



Figure 1: Planting hardiness zones calculated for 1990 from the U.S. National Arboretum From the documentation of this map. Introduction
This map supersedes U.S. Department of Agriculture Miscellaneous Publication 814, "Plant Hardiness Zone Map," which was revised in 1965. This 1990 version shows in detail the lowest temperatures that can be expected each year in the United States, Canada, and Mexico. These temperatures are referred to as "average annual minimum temperatures" and are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico. The map shows 10 different zones, each of which represents an area of winter hardiness for the plants of agriculture and our natural landscape. It also introduces zone 11 to represent areas that have average annual minimum temperatures above 40 F (4.4 C) and that are therefore essentially frost free.


Last year an updated version of this map was published by the Arbor Day Foundation. This new map got a lot of attention because all the zones moved reflective of persistent warmer weather. That is, planting dates previously appropriate for Kentucky are now appropriate for Indiana and Ohio. There are links on this page that show how the fields have changed (Animation link).

There is a lot of evidence that the Earth is warming. And this warming is persistent enough that it is beginning to change biological activity. Spring is coming earlier and Fall is coming later. One of the papers that I use to introduce the subject is by Gian-Reto Walther and many co-authors, entitled, The Ecological Responses to Recent Climate Change. This paper appeared in Nature in March of 2002. Below is a plot of phenology from this paper. Phenology is the occurrence of natural events, and in this case the phenology is associated with the start of spring.



Figure 2: The start of spring from The Ecological Responses to Recent Climate Change


This figure is from observations taken in Germany. Along the horizontal axis are years, and on the vertical axis on the left is the number of days earlier (-) or later (+) that a certain event has occurred. On the right vertical axis is a measure of temperature and the North Atlantic Oscillation Index. The different lines are Temperature (March, April, May), Temperature (March and April); spring arrival of birds and hatching in flycatchers, and the unfurling of leaves in two types of trees (Betula Pendula (a birch) and Aesculus hippocastanum (horse chestnut)) . The yellow line on the figure is the North Atlantic Oscillation (NAO) index, which is a measure of atmospheric variability. (Here is a link to NAO data.)

Spring is coming earlier. Over this set of observations spring is 2-3 days earlier in 2000 that it was in 1950. One thing to point out in the graph is that since 1985 the curves have moved steadily towards the earlier onset of spring. (Note this is the same time of the observations that were used in the update of the hardiness zones by the Arbor Day Foundation (above)).

Of interest to the weather people amongst us is the link to the North Atlantic Oscillation. (Here is a blog for last year on the NAO.) The NAO is a natural model of variability that impacts the temperature in Europe. These natural modes raise the level of “noise” when trying to determine if there is a long-term temperature trend. To make it more difficult, these natural modes, which have a cyclic character, are not independent of carbon dioxide related global warming. They will likely change.

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